Generally, a convoy travel of vehicles is controlled by a technique disclosed in, for example, a patent document 1 (i.e., Japanese Patent Laid-Open No. H11-328584), in which a plurality of vehicles form a convoy of vehicles in a single lane of a road. The convoy of vehicles is controlled by a vehicle travel control which is communicated from vehicles in a front part of the convoy to vehicles in a rear part of the convoy. Such a technique contributes to a reduction of the inter-vehicle distance in the convoy, and as a result, alleviates congestion on the road, for example.
However, when two or more convoys travel in parallel on a multi-lane road, such a technique is unaware of non-convoy vehicles that wish to overtake and pass the two or more convoys. That is, when two or more convoys and an overtaking vehicle(s) are traveling in the same traffic direction on a multi-lane road that has multiple lanes on each side of traffic, according to the convoy travel technique in patent document 1, the non-convoy vehicle may be unable to overtake the two or more convoys traveling in parallel.
More practically, when all lanes of a multi-lane road are obstructed by in-parallel traveling convoys, an overtaking vehicle(s) may be unable to pass such convoys. Similarly, when two lanes of a three-lane road are occupied by in-parallel traveling convoys, the overtaking vehicle may be required to change multiple lanes in order to pass both in-parallel traveling convoys.